Nitrobenzene is an important intermediate of the chemical industry, which is needed in particular for the production of aniline and hence for the production of methylene diphenyl diisocyanate (MIN) and of the polyurethanes based thereon.
The nitration of benzene with nitric acid to give a crude nitrobenzene has already been the subject of numerous publications and patent applications. The methods in common usage today correspond substantially to the concept of the adiabatic nitration of benzene with a mixture of sulfuric and nitric acid (known as a mixed acid). Such a method was first claimed in U.S. Pat. No. 2,256,999 and is described in its modern embodiments in EP 0 436 443 B1, EP 0 771 783 B1 and U.S. Pat. No. 6,562,247 B2, for example. The methods involving adiabatic reaction control are characterised in particular by the fact that no technical measures are taken to introduce heat into or to dissipate heat from the reaction mixture.
Isothermal methods for the nitration of benzene with mixed acid are also described, for example in EP 0 156 199 B1.
Methods for the nitration of benzene that manage without the use of sulfuric acid are described for example in U.S. Pat. No. 2,739,174 or U.S. Pat. No. 3,780,116.
Gas-phase methods for the nitration of benzene with nitric acid or nitrogen oxides are also possible in principle, but the yields that can be obtained with them are still low (EP 0 078 247 B1, EP 0 552 130 B1).
Crude nitrobenzene still contains as impurities water, benzene, nitrophenols and dinitrobenzene and—if nitrated with mixed acid—sulfuric acid, which are separated off by means of suitable processing methods such as scrubbing and distillation stages. A possible embodiment of this processing is described in EP 1 816 117 B1, where the nitrobenzene undergoes an acid scrubbing stage, an alkaline scrubbing stage, a neutral scrubbing stage and finally purification by distillation.
The processing of the alkaline waste water from the alkaline scrubbing stage is described in EP 1 593 654 A1, in which waste waters comprising aromatic nitro compounds are treated at elevated temperature and pressure in a method of thermal pressure decomposition. The waste waters treated by this method can be supplied directly to a biological sewage treatment plant without dilution.
The nitrogen oxides (NOx gases) formed during nitration can be treated with lye and washed out as sodium nitrate and nitrite as described in U.S. Pat. No. 5,313,009. In addition, carbon dioxide, which is formed in the nitration process, is also bound as sodium carbonate.
U.S. Pat. No. 5,963,878 discloses a method in which NOx gases obtained from strategic areas of the nitration system are brought into contact with air and water, for example in a unit having a packed bed, at elevated temperatures and under pressure, wherein the NOx gases are absorbed by the water, forming weak nitric acid. The weak nitric acid is returned to the reaction process. Carbon dioxide is not absorbed in an NOx gas scrubbing tower if the gas-scrubbing tower is operated in an acid mode. Clean, NOx-free exhaust gas is released from the packed-bed unit into the environment.
In all of the referenced methods, therein no further treatment of the waste gases from nitration plants provided.
In WO2012013678A2 it is reported that countless studies in the past have aimed at improving the quality of the crude nitrobenzene and hence increasing the yield of benzene and nitric acid. Thanks to these developments, the modern adiabatic liquid-phase methods have advanced to such an extent that they all succeed in producing a crude nitrobenzene having a low content of secondary products, i.e. on average only between 100 ppm and 300 ppm of dinitrobenzene and between 1500 ppm and 2500 ppm of nitrophenols, wherein picric acid can make up a proportion of 10% to 50% of the nitrophenols.
“Organic Chemical Manufacturing, Volume 7, Selected Processes”, published by the United States Environmental Protection Agency, discloses a method for purifying waste gases from a nitration process, in which the waste gases are purified in an absorption tower with nitrobenzene. No details are given of how nitrobenzene is distributed in the absorption tower. Furthermore, it is disclosed only that nitrobenzene from the nitrobenzene stripper can be used as the scrubbing solution. No details are given of the purity of this nitrobenzene.
DE-OS-29 21 487 discloses a method for removing volatile aromatic compounds from waste gases possibly containing nitrogen oxides and nitric acid, by treating the waste gases with a liquid nitration agent, for example a mixture of nitric and sulfuric acid. The use of nitrobenzene as a scrubbing solution is not disclosed.
When operating a nitration plant for producing nitrobenzene from benzene according to the prior art, it can be established that the isolated molar amount of the end product nitrobenzene and of the secondary products dinitrobenzene and nitrophenol that are inherent to the process is lower than the corresponding amount of benzene used. The outlet for this loss is clearly the waste gas from the production plant, which is undesirable from an ecological perspective. The loss is moreover also associated with economic disadvantages.
There was therefore a need to improve the existing nitration method to the effect that environmental pollution with organic components from the waste gas is reduced. It was moreover desirable to reduce environmental pollution in such a way that there are economic advantages associated with the reduction. It was desirable in particular to recycle benzene entrained in the waste gas.